I ain’t no engineer on this, in fact, I don’t even play one on television, but I can speak from experience in the baking industry where this has been tried/done. When you extract heat from the exhaust gas going up the stack you cool it, causing acids to condense out from the gas. In this condensed form, those acids are extremely corrosive and can drain back into the oven causing extensive damage to both the oven and the exhaust system. This has to be taken into account when designing a heat extractor/exchanger for any oven stack. I’d leave the oven stack alone and go the tankless hot water heater route. I’m even looking into installing one in my home this summer to replace my aging hot water tank.
Tom Lehmann/The Dough Doctor